The present invention relates to a process for preparing from 5 to 60% strength by weight aqueous alkaline solutions of reduced indigoid dyes.
Indigoid dyes, especially indigo itself and its bromine derivatives such as Brilliant Indigo, are well known vat dyes, which are used for dyeing cellulosic textile material. Indigo is particularly important for dyeing cotton warp yarns for blue denim articles.
For dyeing, the water-insoluble dye first has to be reduced (vatted) to convert it into the water-soluble leuco form which has affinity for fiber and which, after going onto the material to be dyed, is then oxidized back to the water-insoluble vat dye pigment.
In conventional indigo dyeing processes, the indigo is reduced in a vessel upstream of the dyebath in an alkaline medium by using sodium dithionite (hydrosulfite) as reducing agent. This method usually produces 8% strength by weight leuco indigo solutions, and the dyehouse wastewater contains large amounts of sulfate and sulfite.
To avoid these disadvantages, a dyeing process was developed to utilize leuco indigo produced by catalytic hydrogenation (WO-A94/23114). As described in prior German Patent Application 198 31 291.1, catalytic hydrogenation in conjunction with certain process measures can be used to produce leuco indigo solutions having a leuco indigo content of up to 55% by weight.
WO-A-90/15182, 95/07374 and 96/32445 disclose the electrochemical reduction of indigo in the presence of large amounts of a mediator (around 4.5% by weight of iron, based on the indigo quantity) in very dilute solution (up to 0.5% strength by weight).
It is an object of the present invention to provide more concentrated indigoid dye solutions which are advantageously useful for dyeing.
We have found that this object is achieved by a process for preparing a from 5 to 60% by weight aqueous alkaline solution of a reduced indigoid dye, which comprises reducing said indigoid dye electrochemically in the presence of a mediator.
The mediator can be any redox system known for electrochemical dye reduction.
Examples are mediators based on quinonoid compounds such as anthraquinonesulfonic acids and hydroxyanthraquinones, e.g., 1,2-dihydroxy-9,10-antraquinone, and on quinoneimines and diimines and also mediators based on transition metal complex salts.
Among transition metal complex salts, iron(II/III) complex salts are particularly important.
Examples of particularly suitable complexing agents are triethanolamine, triisopropanolamine and bicine and the aliphatic hydroxy compounds described in DE-A-43 20 867, which contain at least two coordination-capable hydroxyl groups and may additionally contain up to two aldehyde, keto and/or carboxyl groups, in which connection compounds derived from sugars are of particular interest. Specifically, the following examples are preferred:
di- and polyalcohols such as ethylene glycol, diethylene glycol, pentaerythritol, 2,5-dihydroxy-1,4-dioxane, especially sugar alcohols such as glycerol, erythritol, hexitols such as mannitol, dulcitol, sorbitol and galactitol;
di- and polyhydroxyaldehydes such as glyceraldehyde, triose reductone, especially sugars (aldoses) such as mannose, galactose and glucose;
di- and polyhydroxyketones such as, in particular, sugars (ketoses) such as fructose;
di- and polysaccharides such as sucrose, maltose, lactose, cellulose and molasses;
di- and polyhydroxymonocarboxylic acids such as glyceric acid, particularly acids derived from sugars, such as gluconic acid, heptagluconic acid, galactonic acid and ascorbic acid;
di- and polyhydroxydicarboxylic acids such as malic acid, particularly sugar acids such as glucaric acids, mannaric acids and galactaric acid;
trihydroxycarboxylic acid such as citric acid.
It will be appreciated that mixtures of complexing agents can also be used.
Particular preference is given to monocarboxylic acids derived from sugars (especially gluconic acid and particularly heptagluconic acid) and their esters, lactones and salts.
The molar ratio between complexing agent and iron cation is customarily in the range from 15:1 to 5:1 in the case of iron(II/III)/triethanolamine complexes and generally in the range from 3:1 to 0.5:1, especially in the range from 1.5:1 to 0.5:1, in the case of complexes with the complexing agents described in DE-A-43 20 867, especially heptagluconic acid.
Iron(II/III) complex salts can be prepared by mixing iron(II) or (III) salts such as iron sulfate, iron acetate or preferably iron chloride with the complexing agent or one of its salts in the presence of water. The starting compounds can be used in the form of a solid or in the form of an aqueous solution. If an iron(III) salt is used, the iron(III) is preferably reduced to the active iron(II) complex at the start of the electrolysis.
The mediator is preferably used in the process of the invention in the form of an alkaline solution from about 5 to 8% strength by weight.
Surprisingly, the process of the invention needs a significantly smaller amount of mediator to reduce the indigoid dye than the above-described processes for electrochemical dye reduction. For instance, the amount used in the case of a mediator which transfers one electron per molecule is customarily in the range from 0.003 to 0.08 mol, preferably in the range from 0.008 to 0.05 mol, of mediator per mole of dye.
This small amount of mediator has little if any effect on the solubility of the reduced dye. Therefore, the mediator does not have to be removed from the ready-prepared leuco dye solution.
Advantageously, in the process of the invention, only a portion of the total amount of dye to be reduced is precharged to the electrolysis cell and the remainder is added a little at a time (in power or paste form) during the reduction. However, the addition of dye need not be commenced until after the current has been switched on. This makes it possible to produce even highly concentrated leuco dye solutions (especially  greater than 20% strength by weight leuco indigo solutions) without problems, which is not possible with reducing agents such as hydrosulfite or by hydrogenation.
It is particularly advantageous for viscosity reasons for the concentration of unvatted dye in the electrolysis bath not to exceed 25% by weight, especially 20% by weight.
If cathodes of carbon or graphite felt are used (and they permit a particularly high current yield), it is advisable to further lower the solids content of the electrolysis bath (and keep it as low as possible) so as to prevent blinding of or mechanical damage to the electrode material.
The reduced dye is soluble in an aqueous alkaline medium in the form of a leuco salt. Up to 20% by weight leuco indigo solutions are stable in the presence of one alkali metal hydroxide as alkali. At up to 80xc2x0 C. this is true for up to 40% by weight solutions.
To keep the reduced dye in stable solution at relatively high concentration (generally  greater than 20% by weight) even at room temperature, it is advisable to convert the reduced dye into a mixed alkali metal salt. Accordingly, the alkali used for more concentrated solutions is preferably a mixture of at least two alkali metal hydroxides wherein none of the alkali metal hydroxides accounts for more than 70 mol %.
Preferred alkali metal hydroxides are lithium hydroxide, sodium hydroxide and potassium hydroxide, which may all be present as a triple mixture or in the form of the respective double combinations, in which case mixtures of sodium hydroxide and potassium hydroxide are particularly preferred.
In mixtures of two alkali metal hydroxides, the molar ratio between the two is preferably in the range from 2:1 to 1:2, particularly preferably in the range from 1.5:1 to 1:1.5, most preferably about 1:1.
It is further advisable to minimize the electrolyte excess in order that any salting out of the dissolved leuco dye may be prevented.
The alkali, especially the alkali metal hydroxide mixture, is therefore generally used in amounts of from 1.2 to 2 mol, preferably of from 1.4 to 1.8 mol, in particular of around 1.5 mol, per mole of leuco dye.
The process of the invention is customarily carried out at from 10 to 80xc2x0 C. Higher temperatures  greater than 40xc2x0 C. increase not only the rate of reduction but also the solubility of the reduced dye. Thus the use of alkali metal hydroxide mixture and avoidance of an electrolyte excess will produce at about 40-60xc2x0 C. from 50 to 55% strength by weight and at 60-80xc2x0 C. from 55 to 60% strength by weight leuco indigo solutions devoid of any tendency to crystallize out.
The process of the invention may be carried out using the electrolysis cells disclosed in WO-A-90/15182, 95/07374 and 96/32445.
The process of the invention is useful for producing concentrated leuco salt solutions of all indigoid dyes, for example of indigo itself, mono-, di-, tri and tetrabromo- and/or -chloro-indigo and thioindigo and its derivatives.
The process of the invention is preferably used for producing from 15 to 45% strength by weight leuco dye solutions, in particular from 20 to 30% strength by weight leuco dye solutions, the corresponding leuco indigo solutions being of particular interest.
The leuco dye solutions obtained according to the invention are advantageously useful in indigo dyeing installations for all dyeing processes, for example yarn dyeing, exhaust dyeing, garment dyeing.